Systems for supplying machining energy from a machining electric source to the electrodes of a wire cutting electric discharge machine can be roughly classified into a first group of systems in which a capacitor is connected between an electrode and a workpiece and the capacitor thus connected is charged through a switching transistor which is turned on and off, so that electric discharge machining is carried out by utilizing the voltage of the capacitor thus charged, and a second group of systems in which the current between the electrodes is directly controlled on and off by a switching circuit connected between the power source and the electrodes.
In the first group of systems, as shown in FIG. 1, a capacitor 14 is connected between a wire electrode 10 and a workpiece 12, and machining energy from a power source 16 is supplied through a current limiting resistor 18 and a switching transistor 20 to the capacitor 14 thus connected. The capacitor 14 is charged through the switching transistor 20 which is turned on and off by an on-off pulse signal outputted by an oscillator 22. Thus, pulse voltage and current are applied between the wire electrode 10 and the workpiece 12 by the capacitor 14 thus charged.
The parts (a) and (b) of FIG. 2 are waveform diagrams respectively showing an interelectrode voltage V and a machining current I in the electric discharge machine in FIG. 1. The waveform of the interelectrode voltage V is defined by the time constant CR which is determined from the capacitance C of the capacitor 14 and the resistance R of the resistor 18. In the machine shown in FIG. 1, the timing of occurrence of electric discharge between the electrodes depends on the gap between the electrodes, the specific resistance of the machining solution between the electrodes and the presence of powder which is created during electric discharge machining. Therefore, for instance, electric discharge starts before the charge voltage of the capacitor 14 reaches the supply voltage Vcc, or with a delay time after it reaches the interelectrode voltage. In this case, the peak value Ip and the pulse width .tau.p of the current between the electrodes are as follows: ##EQU1## In these equations, E is the discharge start voltage, and L is the inductance of the current path. Accordingly, in the machine in FIG. 1, the machining current is determined by the voltage E which is provided at the start of electric discharge. Therefore, the machining current value is not constant for every electric discharge. On the other hand, as the amount of a part of the workpiece, which is removed by one discharge machining operation depends on the value of current which is provided during electric discharge, the machined surface roughness is determined by the maximum value of current during electric discharge. In general, as a discharge current is increased, the discharge machining speed is increased. However, the wire cutting electric discharge machine in FIG. 1 suffers from a difficulty that, since the machining current is not constant for every electric discharge, the machining speed for a given surface roughness is decreased.
One example of the second group of systems has been disclosed by Japanese Patent Application Publication No. 13195/1969. It will be briefly described. The system has a main switching circuit for supplying a current large in peak value between the electrodes, and an auxiliary switching circuit which is small in current peak value and is used only to supply a voltage across the electrodes. First the auxiliary switching circuit applies the voltage across the electrodes, and after the occurrence of electric discharge has been detected, the main switching circuit is closed for a predetermined period of time to apply current as desired. Accordingly, as shown in the parts (a) and (b) of FIG. 3, discharge current I uniform with respect to an interelectrode voltage V can be formed so as to be supplied between the electrodes. Therefore, the machining speed for a given surface roughness can be increased, so that the above-described difficulty accompanying the first group of systems using the capacitor is eliminated. However, the second group of systems still suffers from the following drawbacks:
The first drawback is that the desired machining current flows even during abnormal electric discharge (for instance when there is no no-load time in which electric discharge is not started even by application of voltage; i.e., the electric discharge is started upon application of the voltage). The second drawback resides in that, since the same current flows even when short-circuiting occurs between the wire electrode and the workpiece, the wire electrode, etc. may be broken by Joule heat. These will be described in more detail. The average current I during machining is as follows: ##EQU2## where T.sub.ON is the period of time for which the main switching circuit is closed to supply the current, T.sub.OFF is the pause time for which application of the voltage is suspended, Ip is the peak current, and T.sub.OPEN is the average no-load time. On the other hand, the average current I' during abnormal electric discharge is: ##EQU3## Therefore, the current increase rate I/I' during abnormal discharge is: ##EQU4##
In general, in a wire cutting electric discharge machining operation, the difference between (T.sub.OFF +T.sub.ON) and (T.sub.OPEN) being large, (T.sub.OPEN) should be two to three times (T.sub.OFF +T.sub.ON) or more, and therefore the current I' during abnormal discharge is two to three times the current I or more. However, as the current which can flow in the wire electrode is limited, the wire electrode may be broken during abnormal discharge.
As is apparent from the above description, the second group of system can eliminate the drawback accompanying the first group of systems; however, it suffers from the drawback that the wire electrode is broken when abnormal conditions occur--for instance when, with a wire cutting electric discharge machine, a straight-line machining operation is switched over to a pattern machining operation (the wire being most frequently broken in this case), or when the operation of the interelectrode voltage servo mechanism is not suitable, or when the interelectrode servo does not operate quickly because of the large vibration of the wire electrode.